An aircraft is propelled by two or more turbojets, each housed in a nacelle which also encloses a series of associated activating devices connected to its operation and performing various functions when the turbojet is running or stopped. These associated activating devices may include in particular a mechanical system for operating thrust reversers.
A nacelle usually has a tubular structure comprising an air inlet forward of the turbojet, a middle section designed to surround a fan of the turbojet, and a rear section which encloses thrust reverser means, is designed to surround the turbojet combustion chamber, and usually ends in an exhaust nozzle whose outlet is situated downstream of the turbojet.
Modern nacelles are often designed to enclose a turbofan capable of generating by means of the blades of the rotating fan a hot airflow (also called the primary flow) leaving the turbojet combustion chamber.
A nacelle usually has an outer fixed structure (OFS), which defines, with an inner concentric structure of the rear section, termed the inner fixed structure (IFS), surrounding the structure of the turbojet proper to the rear of the fan, an annular flow channel, also termed the secondary stream, designed to channel a secondary flow of cold air around the outside of the turbojet.
The fixed inner structure and the exhaust nozzle define an engine compartment ventilation outlet section whose main purpose is to renew the air flowing between the IFS and the engine, but which can be used to recover some of the thrust lost by the air taken from the secondary stream by controlling the geometry of the cross-sectional area of the airflow. For this purpose, spacer means in the form of rigid spacers are positioned in the outlet section and held in place by belting.
In flight, however, the turbojet has a tendency to twist, in a different way from the nacelle. This causes very large loadings of said engine compartment on the nacelle.